Generally, with the aim of transmitting digital data through a transmission channel, these data are modulated using for example a modulation of the pulse amplitude type ("Pulse Amplitude Modulation" or "PAM"). Quadrature amplitude modulations or QAM are used to increase the sum of the data which can be transmitted within a bandwidth of an available channel. QAM modulation is a form of PAM modulation in which a plurality of information bits are transmitted together in an arrangement subsequently referred to as a constellation.
With a view to synchronizing with the signal received, the digital receiver must be provided with a device for generating a reference signal in phase with the signal received. Having been synchronized, the demodulator allows the demodulation of signals containing information in their phase. For example, in QAM modulation, the modulation of "0" and "1" bits corresponds to phases, in the modulated signal, which are determined according to rules which are known per se. Thus, the demodulator must generate a reference signal which must be synchronized in phase with the data carrier. This process is known by the name of carrier phase recovery.
In PAM modulation, each signal is a pulse whose amplitude level is determined by a transmitted symbol. In QAM modulation, for example in 16-QAM modulation, the amplitudes of the symbols -3, -1, 1 and 3 in each quadrature channel are used. It happens that the effect of each symbol transmitted through a channel extends beyond the time interval used to represent this symbol. The distortion caused by the resulting overspill of the symbols received is termed intersymbol interference (or ISI). This distortion has been one of the principal obstacles for data transmissions at high bit rate on limited bandwidth noisy channels. A device known as an "equalizer" is then used to remedy this intersymbol interference problem.
With the aim of reducing the intersymbol interference introduced by the transmission channel, accurate equalization is required. Since the characteristics of the channel are not known in advance, a statistical equalizer is thus used which carries out a mean compensation of the domain of the channels required in terms of amplitude and delay characteristics. The mean square error stochastic gradient algorithm, also known as the LMS algorithm (standing for Least Mean Squares) is generally used as adaptive equalization algorithm.
Thus, one of the essential functions of the receiver in digital transmission systems is therefore the extraction of a carrier synchronized in phase and in frequency with the carrier at the transmission end. A poor phase or a poor frequency at demodulation level reduces the power of the useful signal and creates interference between the quadrature components I and Q of the demodulated QAM signal, thus explaining the importance of the recovered phase.
Another essential function is also, as seen above, the elimination of the distortions of the signal received. Moreover, the response of the channel generally being unknown and, furthermore, susceptible of variation over time, its equalization then requires an adaptive equalizer capable of adapting itself to the channel and of tracking its temporal variations.
Now, in conventional receivers, the adaptive equalizer and the carrier recovery device, comprising a frequency estimator and a phase estimator which are switched as a function of particular criteria, follow one another in the reception chain. In this context, it is not possible to effect carrier recovery in the presence of echoes of large amplitude or of too considerable a phase shift.